1. Field of the Invention
The present invention relates to an ink jet recording apparatus for image recording by discharging of recording liquid (ink), and more particularly to an ink jet recording apparatus equipped with a recording head having plural orifices with an improved recovery system for orifice clogging caused by dust, or defective discharging from the orifice caused by viscosity increase of ink or presence of bubbles therein.
2. Related Background Art
FIG. 1 is a schematic perspective view showing an example of the recording head employed in the ink jet recording apparatus, wherein a discharging element 1 is provided with liquid paths in which respectively arranged are heat generating elements constituting means for generating thermal energy utilized for the ink discharging, discharging openings 10 provided at the front ends of said liquid paths and a common liquid chamber for storing ink to be supplied to said liquid paths, and discharges ink from said discharging openings to form recording liquid droplets.
There are further shown a base plate 3 for fixing the discharging element 1 for example with an adhesive; a front plate 2 fixed on an end face of the discharging element 1 and the base plate 3 for example with bolts and having an aperture 2a for maintaining the discharging openings 10 in direct facing relationship to a recording medium; and members 15, 16. 17 constituting a part of an ink supply system, in which 15 is a connecting elbow pipe for introducing ink to the common liquid chamber in the discharging element 1, 17 is a filter unit provided in the ink supply path from an ink source such as an ink tank, and 16 is a supply pipe connecting the member 15 with the filter unit 17.
FIGS. 2 and 3 are respectively vertical and horizontal schematic cross-sectional view of the recording head shown in FIG. 1, wherein a cap 4 is pressed to the face of discharging openings of the discharging element 1 across the front plate 2 (omitted in FIGS. 2 and 3) for the recovery of discharging failure.
The liquid paths 12 respectively corresponding to plural discharging openings 10 communicate with a so-called canopy portion 13, which in turn communicates with a common liquid chamber 14. Energy generating means 11, for example composed of a heat generating element is provided in the liquid path 12 for the purpose of generating energy utilized for ink discharging. Inside the filter unit 17 there is provided a filter 100 composed of a mesh for eliminating small dust particles and bubbles.
FIG. 4 is a schematic view of a discharge failure recovery system in the conventional ink jet recording apparatus. In the normal recording state, a cap 4 is placed in a position not hindering the recording operation, and the ink is supplied from the ink tank 6 to the discharge element 1 by capillary action.
At the recovery of discharge failure, the cap 4 is fitted on the discharge element 1 in air-tight manner, and a pump 7 is actuated in this state to generate a negative pressure inside the cap 4 in comparison with the ink tank 6, thereby forcibly sucking the ink from the discharging opening 10. At the same time, the dust, viscous ink, bubbles etc. responsible for the discharge failure are removed from the discharge element 1, together with the sucked ink. For example a minute bubble a that has migrated into a liquid path 12 as shown in FIG. 3 can be removed through the discharging opening 10 together with the ink, by the actuation of the pump 7. The ink removed from the discharging opening 10 is received by the cap 4 and guided to a used ink tank 5.
FIG. 5 is a circuit diagram showing a fluid chemical equivalent circuit for the ink in the discharge failure recovery in the conventional apparatus. At the discharge failure recovery, the following relation exists. EQU .DELTA.P=qR1+nq(RH+RC+RF+RS) (1)
among the suction force .DELTA.P, number of liquid paths n, fluid resistance R1 per each liquid path, fluid resistance RH of the canopy 13, fluid resistance RC of the common liquid chamber 14, fluid resistance RF of the filter 17, fluid resistance RS from the ink tank 6 to the common liquid chamber 14 except the filter 17, and flow rate q of the liquid path 12 with a suction force .DELTA.P. Thus: EQU q=.DELTA.P/{R1+n(RH+RC+RF+RS)} (2)
Usually the supply system is so designed as to obtain a relation R1 RH+RC+RF+RS, but, in a so-called full-line multiple head in which the discharging openings respectively communicating with the liquid paths are arranged by a number corresponding to the full recording width, the number n of the liquid paths becomes very large, so that the flow rate q per liquid path becomes very small. Also in case bubbles or dust particles enter the liquid path 12 as shown in FIG. 3, the fluid resistance of said liquid path becomes higher. Consequently the flow rate in the liquid path with discharge failure becomes even lower than in the normal liquid path.
Let us consider a case in which a bubble has entered a liquid path 12. Since the bubble usually sticks to the wall of the liquid path, there is required a pressure change or a flow rate in the liquid path 12, in order to peel the bubble off the wall. However, in such conventional structure, since the fluid resistance is smaller in the normal liquid paths, the pressure change obtained in the liquid path with discharge failure becomes even smaller.
FIG. 6 is a schematic perspective view showing another example of the recording head employed in the conventional ink jet recording apparatus, and FIGS. 7 and 8 are respectively a vertical and horizontal schematic cross-sectional views of the recording head shown in FIG. 6.
The present example differs from the foregoing example in that the discharging element 1 is provided with two supply pipes 16, and that the recovery of discharge failure is conducted with a pressure applied to the ink in the supply pipes 16.
In the present example, receiving member 4a is provided for receiving the ink expelled from the discharging openings by the pressure.
FIG. 9 is a schematic view of a discharge failure recovery system in the ink jet recording apparatus of the present example. In the normal recording state, a receiving member (cap) 4a is placed at a suitable position not hindering the recording operation, and a valve B2 is closed while valves B1, B3 are opened, whereby the ink is supplied from an ink tank 6 to a discharging element 1 through the valve B1 by capillary action.
At the discharge failure recovery, the cap 4a is fitted on the discharging element 1, and the valve B1 is closed while the valves B2, B3 are opened. In this state a pump 7 is actuated to send the ink from the ink tank 6 to the ink supply path under pressure, thereby supplying the discharging element 1 with pressurized ink and forcedly ejecting ink from the discharging openings 10. At the same time, the dust, viscous ink, bubbles etc. responsible for the discharge failure are removed from the discharging element 1, together with the ejected ink. For example a minute bubble a that has migrated into a liquid path 12 as shown in FIG. 8 can be removed through the discharging opening 10 together with the ink, by the actuation of the pump 7. The ink removed from the discharging opening 10 is received by the cap 4a and guided to a used ink tank 5.
Now reference is made to FIG. 5 for explaining the fluid mechanical equivalent circuit in the present prior art. At the discharge failure recovery, the following relation exists EQU .DELTA.P=qR1+nq(RH+RC+RF+RS) (1)
among the pressure .DELTA.P, number n of liquid path, fluid resistance R1 per each liquid path 12, fluid resistance RH of the canopy 13, fluid resistance RC of the common liquid chamber, fluid resistance RF of the filter 17, fluid resistance RS from the ink tank 6 to the common liquid chamber 14 except the filter 17, and flow rate q of the liquid path 12 under the pressure .DELTA.P. Thus: EQU q=.DELTA.P/{R1+n(RH+RC+RF+RS)} (2)
Usually the supply system is so designed as to obtain a relation R1 RH+RC+RF+RS, but, in a so-called full-line multiple head in which the discharging openings respectively communicating with the liquid paths are arranged by a number corresponding to the full recording width, the number n of the liquid paths becomes very large, so that the flow rate q per liquid path becomes very small. Also in case bubbles or dust particles enter the liquid path 12 as shown in FIG. 8, the fluid resistance of said liquid path becomes higher. Consequently the flow rate in the liquid path with discharge failure becomes even lower than in the normal liquid path.
Consequently the conventional recovery system for discharge failure has been often unable to restore the defective liquid path to the normal state, or has to repeat the recovery operation to realize such normal state.
Also there has been required a large suction force or pressure in order to overcome such fluid resistance, and a large pump 7 has been required for this purpose. This has resulted in a larger consumption of ink, and, particularly in case of a recovery operation with an increased pressure, a need for an increased junction strength for withstanding such pressure.